1. Field of the Invention
This invention relates to the field of infrared sensing, and more particularly, to an integrated infrared detector and cryoengine assembly which does not require evacuation for operation.
2. Description of Related Art
Infrared detectors are often used in conjunction with missiles and night vision systems to sense the presence of electromagnetic radiation having a wavelength of 1-15 .mu.m. Because many of them are most sensitive when operating at approximately 77.degree. K., infrared detectors fabricated from mercury-cadmium-telluride often require a cryoengine assembly to produce and maintain the required operating temperature. Such cryoengine assemblies are typically used in conjunction with an evacuated dewar in which the infrared detector is placed. The dewar is evacuated to remove gases which would otherwise occupy the region surrounding the detector so that potential heat loss through convection and conduction is minimized. The dewar is cooled by placing an indented region ("coldwell") of the dewar in contact with an expansion chamber ("coldfinger") of the cryoengine assembly which is suppplied with a cooling fluid such as helium. When used with a split cycle cryoengine, the coldfinger is referred to as an expander. To supply the expander with the cooling fluid, the cryoengine assembly includes a compressor which delivers the fluid to the expander by means of a transfer line. As the fluid expands in the expander, it absorbs thermal energy from both the expander and the dewar causing the detector to cool. Because the temperature of the expander is related to the amount of fluid delivered by the compressor, the temperature of the detector may be controlled by varying the compressor speed.
While the infrared detector assemblies described above are able to produce and maintain the temperature levels required for sensitive operation of the mercury-cadmium-telluride detectors, design constraints often limited material selection, fabrication methods, and closure techniques. To maintain the vacuum within the dewar, the materials from which the dewar was fabricated had to be relatively free from vacuum leakage and have low diffusivity with respect to environmental gases. The materials also had to be resistant to outgassing, in which environmental gases would be released from the materials while the dewar was being evacuated. After the appropriate selection of materials, the dewar had to be fabricated using techniques which did not reduce their diffusion resistant properties or produce vacuum leaks. Furthermore, implementing the necessary closure techniques required to ensure the vacuum inside the dewar would be maintained were often costly and generally required the use of vacuum pumps, leak detectors and residual gas analysis devices. In addition to design constraints, thermophonic and microphonic difficulties also were often present in the infrared detector assemblies described above.